In a healthy knee, the patella (kneecap) glides smoothly in a groove (called the trochlea) on the lower end of the femur (thigh bone) as your joint is bent or extended. The patellofemoral joint is supported by a complex network of ligaments, tendons and other soft tissues.
Problems affecting the patellofemoral joint most frequently include anterior knee pain, instability (subluxations or dislocations of the patella, when the kneecap moves partially or fully out of the groove in the femur) and arthritis.
There are many causes for anterior knee pain and it is essential that a thorough assessment is carried out to evaluate the patellofemoral joint but also to look for imbalances that may be present from your hips to the feet. Patellofemoral problems can be aggravated by factors such as flat feet, abnormal hip rotation, tightness of the iliotibial band and hip flexor muscles. It is essential that a precise diagnosis is made to be able to formulate an appropriate treatment plan.
The stability of your kneecap is dependent on several factors. These include the shape of the joint, the ligaments and muscles and the overall alignment of your lower limb bones. When the knee cap dislocates it moves off the front of the thigh bone to sit on the outer side.
Traumatic dislocations of the patella involve a significant force, for example during an impact sustained in sports, or a forceful twisting movement. They usually occur in a normally shaped joint and mostly do not recur.
The term atraumatic dislocation is usually applied if your kneecap dislocates with much less force. This usually implies an underlying abnormality of the shape of your patellofemoral joint such as trochlea dysplasia (shallow or non-existent groove), patella alta (abnormally high kneecap) and tibia and femur torsion (rotational deformity of the bones). Generalised joint laxity can also be a contributing factor. Patients sustaining an atraumatic dislocation may be at risk of recurrent instability.
Acute patella dislocations sometimes need to be manipulated back into joint although the kneecap may relocate spontaneously. Initial treatment will focus on settling the immediate pain and swelling followed by physiotherapy-led rehabilitation to restore muscle strength and control. An x-ray is obtained to exclude a bony injury and this may be supplemented by a MRI scan, particularly if symptoms persist and surgery is being considered. Early surgery is rarely indicated unless there is a loose piece of bone that needs to be fixed or removed.
When symptoms of patellofemoral instability persist despite a physiotherapy programme, surgery may be indicated. The specific procedure performed depends on a detailed assessment and identification of any abnormality that is predisposing to recurrent instability.
Medial patellofemoral ligament (MPFL) reconstruction is an operation performed to reconstruct a fibrous structure that normally runs from the inner aspect of your kneecap to the inner aspect of your thigh bone and provides approximately 60% of the stability to the kneecap in a normally shaped knee. This structure is usually torn when the kneecap dislocates.
Tibial tubercle osteotomy is a procedure performed to detach the tibial tubercle (lump on the font of the shin bone where the patella tendon attaches) and fix it in a different position. This is usually performed if you have a high-riding kneecap or if the tibial tubercle to trochlea groove distance is significantly higher than normal.
If the groove for your kneecap on the front of your thigh bone has failed to develop normally (trochlea dysplasia) and, in particular, if there is a bump rather than a groove, then a highly-specialised procedure called trochleoplasty is sometimes indicated to create a groove.